The present invention relates generally to a conformal coating and methods for conformally coating a substrate to provide corrosion resistance, and more particularly, to a conformal coating and method for conformally coating printed electrical circuit boards and the components mounted thereto to provide corrosion resistance.
Low power, battery operated environments present an environment where electronics are more susceptible to humidity-induced environmental changes. A particularly acute need exists in commercial utility meters such as natural gas meters, water meters or electric meters. Such applications are exposed to temperature extremes of xe2x88x9240xc2x0 F. to 160xc2x0 F., in varying humidity conditions, and sometimes even immersed in water. Additionally, battery life in such commercial utility meters must extend to 10 or more years in certain applications. Given the low power environment inherent in extended battery life applications, the electronics are configured to periodically awaken from an ultra-low power idle state for processing.
The electronics are implemented on printed circuit boards having electronic components such as integrated circuits, crystals, inductors, batteries, resistors, capacitors, diodes, transistors, switches and sockets. Each of these components has leads which mount to the surface of the printed circuit board, as in surface mount applications, or extend through the circuit board as in traditional thru-hole applications. Conformal coatings for printed electrical circuit boards, including the components and the associated leads, and techniques for coating the same have been used to prevent corrosion and shorting from exposure to humid conditions. However, many conformal coatings, in particular, the widely used acrylic-based coatings, often delaminate or pull away from the corners of leads and boards and develop cracks which wick moisture during temperature and humidity cycles. These cracks and areas of delamination form pockets which may entrap water and dissolve and/or disassociate contaminants confined therein. These contaminants may eventually form dendritic growths between component leads, which lead to shorts, excessive power consumption and board malfunctions. These problems are exacerbated in low power, battery operated environments where moisture and dendritic formations between leads project a relatively low impedance, with respect to the high impedance battery application.
Therefore, there remains a need for a conformal coating for printed circuit boards which will provide moisture and corrosion resistance for the printed circuit boards and the components mounted thereto without cracking or delaminating.
The present invention provides circuit boards and their associated components with a protective conformal coating, which is sealed and corrosion resistant, thus eliminating cracks and spaces that would result from incomplete coating coverage or delamination and the influx of water and contaminants that can cause shorting and corrosion of leads. This allows indefinite operation of low-powered electronics in varying moisture environments, or even submerged in water.
The present invention is directed to a protective conformal coating and method for applying the same to printed circuit boards and the components thereon to provide a corrosion resistant, sealed, stratified coating which eliminates cracks and delamination, thereby providing corrosion and moisture resistance when exposed to humid conditions. Further, the present invention provides a protective conformal coating for circuit boards which allows for repair and reapplication or recovering of the board, components, and component leads.
Preferably, the protective conformal coating comprises a first coating layer of a material selected from the group mainly consisting of parylene, urethane, acrylics, epoxies and silicones which is deposited onto an ultra-clean printed circuit board having a multiplicity of components. Although all of these conformal coating materials are believed acceptable substitutes for the first coating, preferably, parylene is used for the first coating.
Typically, the ultra-clean circuit board is cleaned to approximately about less than 20 micrograms per square inch NaCl equivalent. For best results, the printed circuit board is cleaned to approximately about less than 5 micrograms per square inch NaCl equivalent. The parylene or equivalent coating which is deposited on the ultra-clean circuit board provides a bonded coating that will not delaminate from the printed circuit board, the mounted components or the component leads. It is important that the first coating layer is deposited to cover most, if not all, of the exposed surfaces of the board, components and leads. Care should be taken to coat between and behind lead surfaces. Further, the first coating layer of parylene is removable and redepositable, thereby allowing for board repair and recoating, as necessary.
Typically, the protective conformal coating further comprises a second coating layer comprising a corrosion inhibiting viscous fluid applied onto the first coating layer, thereby filling in gaps and flexible openings occurring in the first coating layer. The corrosion inhibiting lubricant is capable of withstanding corrosive environments and provides a sealed, stratified coating. Like the first coating, the second coating should cover most, if not all, of the printed circuit board, components and lead surfaces. Where the first coating layer is removed and reapplied, the second coating layer of corrosion inhibiting viscous fluid is also reapplied to reseal the printed circuit board, components, and leads.
Accordingly, one aspect of the present invention is to provide a protective conformal coating for a substrate which is corrosion resistant. It is another aspect of the present invention to provide a conformally coated printed circuit board, including components and leads, which is sealed and corrosion resistant. Still another aspect of the present invention is to provide a method for conformally coating a printed circuit board, including components and leads.